DESCRIPTION (investigator's abstract): Continuous Positive Airway Pressure (CPAP) is the most commonly prescribed treatment for obstructive sleep apnea, but adherence is low. Previous efforts to enhance adherence to CPAP treatment yielded modest improvements, and were not theory driven. Motivational Enhancement (ME) is a theory driven treatment that has been used successfully to improve treatment adherence in other medical conditions. The primary aim of this study is to compare the efficacy of ME and an education intervention (ED) to standard clinical care (SC) for improving adherence to CPAP in patients with moderate to severe OSA. A second aim is to examine the relationship between the amount of CPAP use and changes in cognition, mood, self-reports of daytime sleepiness and sleep-related functional status. Three hundred participants (30-80 years of age) diagnosed with sleep apnea will use CPAP for 1 week and will then be randomized to 1 of 3 conditions. The SC group will receive only the usual clinical care from the sleep center, the ME and ED group will receive two 45-minute individual treatment sessions after 1 week and 2 weeks of CPAP use. The ME intervention will provide each participant with physiological feedback of their sleep and cognitive functioning, and incorporate techniques specifically aimed at enhancing motivation to adhere to treatment. The ED group will receive the same amount of individualized attention, but will only be instructed on the patho-physiology of and correlates of OSA and benefits of treatment. CPAP adherence will be monitored covertly by data recorded in the CPAP units after 2 weeks, 3, 6 and 12 months. Follow up assessments of cognition, mood, daytime sleepiness and sleep-related functional status will be conducted at 3,6 and 12 months post treatment.